New Louvers ensure that Rosetta is real cool
22 March 2000
Imagine leaving home for 11 years, to embark on a trek which will take you from the frozen wastes of Antarctica to the sizzling deserts of Arabia. Working out how to survive such extreme temperature variations would be a major headache.A similar problem is facing the designers of ESA's Rosetta comet chaser. In the early and late stages of its prolonged expedition, the spacecraft will sweep across the inner Solar System, where sunlight is plentiful. However, in order to rendezvous with Comet Wirtanen, Rosetta will have to probe beyond the asteroid belt, more than 5 times the Earth's distance from the Sun. In those frigid regions, the solar energy levels are only 4% of the those that we enjoy on our balmy planet.Since it is not feasible to wrap a spacecraft in multiple layers of warm clothing for periods of deep freeze, then strip these away when sunbathing is the order of the day, the ESA team has been obliged to come up with alternative ways of regulating temperature.
One straightforward solution is to wrap the tonne spacecraft in a blanket of insulating material. But this is only useful during periods of deep freeze, and may lead to overheating when Rosetta is closer to the Sun.
So designers have provided Rosetta with louvers - high-tech Venetian blinds which control the spacecraft's heat loss. Lovingly polished by hand, these assemblies of thin metal blades must be handled like precious antiques, since any scratching, contamination or fingerprints will degrade their heat reflecting qualities.
The principle behind the louvers is quite simple. When Rosetta is cruising around the inner Solar System and basking in the warmth of the Sun, surface temperatures may soar to 130°C, and even internal equipment can reach 50°C. At such times, it is vital to stop the spacecraft from overheating, so the louvers are left fully open, allowing as much heat as possible to escape into space from Rosetta's radiators.
However, during its prolonged deep space exploration and comet rendezvous phases, when temperatures plummet to -150°C, heat conservation is the order of the day. Since the spacecraft's limited internal power supply - equivalent to the output from three ordinary light bulbs - then becomes the main source of warmth, it is essential to trap as much heat as possible. This means completely closing the louvers in order to prevent any heat from escaping.
How efficient is this system? Engineers at the European Research and Technology Centre (ESTEC) in The Netherlands are about to find out. The one and only set of louvers - the same hardware that will eventually fly on board Rosetta to Comet Wirtanen - has now been delivered to ESTEC.
All 14 louvers, covering a total of 2.5 square metres, have recently been fitted over the radiators on two sides of the spacecraft. Starting in late March, they will undergo an exhaustive series of thermal tests in order to simulate the wildly fluctuating temperatures that Rosetta will experience during its 11 year odyssey to the comet. But the ESTEC engineers are confident that, with the aid of its reflective louvers, Rosetta will be the coolest spacecraft around.
